There have been proposed many processes for preparing optically active threonine. The typical one comprises synthesizing DL-threonine and then optically resolving the racemate with an enzyme to obtain the desired isomer, the undesired isomer being reused through racemization, as described in Fodor, et al., J. Biol. Chem., Vol. 178, 503 (1949).
A problem in the conventional synthesis of threonine lies in the difficulty of selectively synthesizing a threo form. That is, the product obtained generally comprises a mixture of a threo form and an erythro form at a ratio of 8 to 2. Another problem is that since each of the threo form and the erythro form comprises optical isomers (antipodes), isolation of the respective desired isomers cannot be achieved without racemic resolution. This means that the whole process involves a step for separating and purifying the isomer and a step for racemizing the undesired isomer, thus leading to an increase of production cost.